Flux-cored wire for gas-shielded Arc welding of heat resisting steel

ABSTRACT

A flux-cored wire for gas-shielded arc welding of heat-resisting steel in the form of a steel tube filled with a flux, which is characterized in that the content of slag-forming agent is 6.10-9.90 mass % (based on the total mass of the wire), said steel tube and said flux all together contain less than 0.20 mass % C, 0.06-1.40 mass % Si, 0.55-1.60 mass % Mn, 0.004-0.090 mass % Cu, 0.004-0.090 mass % Ni, less than 2.60 mass % Cr, and 0.3-1.20 mass % Mo (based on the total mass of the wire), and said flux contains 4.2-8.2 mass % TiO 2 , 0.025-0.25 mass % of metal fluoride (in terms of fluorine), and 0.20-1.50 mass % Mg.  
     The flux-cored wire has both good welding maneuverability and ability to give weld metal with good mechanical properties, such as strength and toughness.

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present invention relates to a flux-cored wire for gas-shielded arc welding of heat resisting steel. More particularly, the present invention relates to a flux-cored wire for gas-shielded arc welding of heat resisting steel which is easy to work and gives weld metal having good mechanical properties such as strength and toughness. The flux-cored wire corresponds to YFM-C, YFCM-C, YFLCM-C, and YF2CM-C specified in JIS Z3318-1991 (flux-cored wire for MAG welding of Mo steel and Cr—Mo steel) or corresponds to A1, A1M, B1, B1M, B1L, B1LM, B2, B2M, B2L, B2LM, B2H, B2HM, B3, B3M, B3L, B3LM, B3H, and B3HM specified in AWS A5.29-1998 (Specification for Low-Alloy Steel Electrodes for Flux-Cored Arc Welding).

[0002] The present invention relates also to a flux-cored wire for gas-shielded arc welding of Cr—Mo steel (represented by ASTM A387 Gr. 2, 11, 22) and Mo steel (represented by ASTM A204 Gr. A). To be concrete, the present invention relates to a flux-cored wire for gas-shielded arc welding of Cr—Mo steel containing 0.25-3.0 wt % Cr and 0.25-1.5 wt % Mo and Mo steel containing 0.25-1.0 wt % Mo. 2. Description of the Related Art

[0003] In the field of heat-resisting steel, gas-shielded arc welding is dominating over shielded metal arc welding because of its high efficiency. Wires for gas-shielded arc welding fall under two categories—solid wires and flux-cored wires. The latter have many advantages over the former. For example, flux-cored wires give rise to less spatter than solid wires and cover the bead surface with slag, producing a bead with a good appearance and shape. In addition, it ensures good welding maneuverability regardless of unfavorable welding positions, such as vertical and overhead positions. Unfortunately, as compared with solid wires, flux-cored wires are more likely to give weld metal poor in mechanical properties, particularly toughness. One reason for this is that in the case of welding with rutile-flux-cored steel wires, molten slag does not float on or separate from molten metal completely, partly remaining as non-metallic inclusions in weld metal, with the result that the oxygen content in weld metal becomes as high as 700-900 ppm (by mass).

[0004] A conventional way to cope with the deterioration of weld metal in toughness due to rutile-flux-cored steel wires is by deoxidation of weld metal or by refinement of microstructure.

[0005] For example, there is disclosed a new flux-cored wire for gas-shielded arc welding in Japanese Patent Publication No. 44159/1984. It is designed to reduce the oxygen content in weld metal by incorporation of Mg, metallic Ti, and Fe—Ti into the flux, thereby improving the toughness of weld metal.

[0006] There is also disclosed a combined wire for gas-shielded arc welding in Japanese Patent Publication No. 6840/1981. It is designed for deoxidation of weld metal and refinement of microstructure by controlling the amount of TiO, TiO₂, B, and B₂O₃ so that weld metal has good toughness even in the case of welding with a large amount of heat.

[0007] There is disclosed another flux-cored wire for gas-shielded arc welding in Japanese Patent Publication No. 13432/1996. It is characterized in that the metal tube and flux contain Ti, Ni, and B as well as a limited amount of nitrogen because it is known that nitrogen has an adverse effect on toughness.

[0008] There is disclosed further another flux-cored wire in Japanese Patent Publication No. 13432/1996 and Japanese Patent Laid-open No. 77086/1993. It contains a trace amount of Nb and V so as to prevent the occurrence of coarse ferrite particles and ferrite band having an adverse effect on strength and toughness.

[0009] The disadvantage of the flux-cored wire disclosed in Japanese Patent Publication No. 44159/1984 is that mere incorporation with Mg and Ti does not reduce oxygen content in weld metal and hence does not improve the toughness of weld metal through reduction of oxygen content. It has another serious disadvantage of causing slag inclusion, more spatter, and poor bead shape in welding.

[0010] The combined wire disclosed in Japanese Patent Publication No. 6840/1981 has a disadvantage of being unable to reconcile welding maneuverability with weld metal having good mechanical properties as in the case of flux-cored wire disclosed in Japanese Patent Publication No. 44159/1984.

[0011] The flux-cored wire disclosed in Japanese Patent Publication No. 13432/1996 inevitably causes air inclusion as wire extension varies during welding. Air inclusion results in an increase in nitrogen content in weld metal, with no noticeable improvement in toughness and electrode usability.

[0012] The flux-cored wire disclosed in Japanese Patent Publication No. 13432/1996 and Japanese Patent Laid-open No. 77086/1993 effectively prevents the occurrence of ferrite particles and ferrite band owing to a trace amount of Nb and V contained therein. However, mere incorporation with Nb and V is not enough to improve strength and toughness. Moreover, incorporation with Nb and V makes slag removal difficult. Hence, it does not achieve compatibility between welding maneuverability and weld metal having good mechanical properties as intended in the present invention.

OBJECT AND SUMMARY OF THE INVENTION

[0013] The present invention was completed to address the above-mentioned problems. It is an object of the present invention to provide a flux-cored wire for gas-shielded arc welding of heat resisting steel which offers good welding maneuverability and gives weld metal having good mechanical properties.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a sectional view showing a test plate produced in an example of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention is directed to a flux-cored wire for gas-shielded arc welding of heat-resisting steel in the form of a steel tube filled with a flux, which is characterized in that the content of slag-forming agent is 6.10-9.90 mass % (based on the total mass of the wire), said steel tube and said flux all together contain less than 0.20 mass % C, 0.06-1.40 mass % Si, 0.55-1.60 mass % Mn, 0.004-0.090 mass % Cu, 0.004-0.090 mass % Ni, less than 2.60 mass % Cr, and 0.3-1.20 mass % Mo (based on the total mass of the wire), and said flux contains 4.2-8.2 mass % TiO₂, 0.025-0.25 mass % of metal fluoride (in terms of fluorine), and 0.20-1.50 mass % Mg.

[0016] According to a preferred embodiment for improved toughness, said steel tube and said flux all together further contain 0.005-0.050 mass % Nb, 0.005-0.050 mass % V, and 0.005-0.020 mass % B, (based on the total mass of the wire), and said steel tube and said flux contain less than 0.015 mass % P and less than 0.015 mass % S.

[0017] According to another preferred embodiment for improved toughness, said steel tube and said flux all together further contain 0.02-0.2 mass % Ti (based on the total mass of the wire).

[0018] A detailed description of the invention follows. The present inventors carried out extensive studies on how the bead shape is affected by welding maneuverability, particularly welding position such as vertical or overhead welding. It was found that a good bead shape depends on both molten metal and slag. In other words, a good bead shape cannot be obtained only by adjusting the viscosity of molten metal or only by changing the amount or constituents of slag. This finding suggests the following to obtain a good bead shape.

[0019] (1) Adjustment of the viscosity and flowability of molten metal by means of Si, Mn, and Mg.

[0020] (2) Optimization of the viscosity and flowability of slag per se by adjusting the amount of slag, the amount of TiO₂ (as a major constituent), and the amount of fluoride.

[0021] It was found that if the above-mentioned two objects (1) and (2) are achieved simultaneously, a good bead shape is obtained regardless of unfavorable welding positions, such as vertical and overhead welding.

[0022] Next, the present inventors carried out extensive investigations on mechanical properties such as strength and toughness. It was found that not only Si, Mn, Mg, and fluorides but also C, Cr, and Mo affect mechanical properties and that Cu and Ni materially affect mechanical properties such as strength and toughness.

[0023] It was found that the arc stability affects the degree of segregation of deposited metal. That is, deposited metal resulting from a flux-cored wire (which lacks arc stability) is subject to segregation of alloy constituents. This segregation promotes the occurrence of ferrite band by post-weld heat treatment (PWHT), thereby deteriorating strength and toughness. Knowing that improvement in arc stability leads to improvement in mechanical properties of deposited metal, the present inventors carried out extensive investigations on the arc stabilizer. Thus, it was found that it is important to optimize the amount of TiO₂ (as a major arc stabilizer) and the amount of fluorides. If this object and the above-mentioned object (1) are achieved simultaneously, it is possible to reconcile welding maneuverability with weld metal having good mechanical properties as intended in the present invention.

[0024] According to the present invention, the flux-cored wire for gas-shielded arc welding of heat resisting steel contains several constituents in specific amounts. The reason for this is explained in the following.

Slag-forming Agent: 6.10-9.90 Mass %

[0025] The term “slag-forming agent” as used in this specification denotes any non-metallic components, such as TiO₂ and fluorides, which characterize the present invention. It also embraces Al₂O₃ZrO₂, SiO₂, CaO, and MgO which are intended for adjustment of the basicity of slag and for fine control of the melting point, viscosity, and flowability of slag. It also embraces K₂O, Na₂O, and Li₂O which are intended for fine control of arc state. They produce synergistic effects with TiO2, Si, Mn, Mg, and fluorides mentioned later, thereby giving rise to a good bead shape regardless of welding positions (such as down hand welding, horizontal fillet welding, vertical welding, and overhead welding). In addition, they give rise to sound weld metal. If the content of the slag-forming agent is less than 6.10 mass % of the total mass of the wire, the amount of slag is not enough to cover the bead surface and hence the resulting bead looks poor. Insufficient slag results in a remarkably convex bead in the case of vertical or overhead welding, presenting difficulties in welding operation. Insufficient slag also adversely affects slag removal, causing weld flaws such as slag inclusion and incomplete fusion. On the other hand, if the content of the slag-forming agent is more than 9.90 mass % of the total mass of the wire, slag constantly covers the molten pool, thereby causing weld flaws such as slag inclusion and incomplete fusion. Therefore, the adequate content of the slag-forming agent should be 6.10-9.90 mass % of the total mass of the wire.

C: Less Than 0.20 Mass %

[0026] C is added to either or both of the metal tube and flux for weld metal to have controlled strength and toughness. If the total amount of C in the metal tube and flux exceeds 0.20 mass % of the total mass of the wire, weld metal has excessively high strength and extremely decreases in toughness and becomes liable to weld flaws such as hot cracking and blowhole. Therefore, an adequate content of C in the metal tube and flux should be less than 0.20 mass % of the total mass of the wire. Incidentally, C should be added to the flux in the form of simple substance (such as graphite) or alloy (such as chromium carbide, Si—C, high C—Fe—Mn, and high C—Fe—Cr).

Si: 0.06-1.40 Mass %

[0027] Si deoxidizes weld metal, thereby controlling its strength and toughness, and optimizes the amount of slag-forming agent. In addition, it adjusts the bead shape when added in combination with Mn, TiO₂, Mg, and fluorides. Therefore, Si should be added to either or both of the metal tube and flux. If the total content of Si in the metal tube and flux is less than 0.06 mass % of the total mass of the wire, Si does not fully produce its deoxidizing effect but causes weld flaws such as blowhole, resulting in weld metal lacking strength and toughness. With an insufficient amount of Si, molten metal lacks adequate viscosity, giving rise to a poor bead shape in the case of vertical or overhead welding. On the other hand, if the total amount of Si in the metal tube and flux exceeds 1.40 mass % of the total mass of the wire, Si coarsens ferrite particles in weld metal at the time of PWHT, thereby greatly deteriorating toughness. Therefore, the total amount of Si in the metal tube and flux should be 0.06-1.40 mass % of the total mass of the wire. If the total amount of Si in the metal tube and flux is more than 0.10 mass % of the total mass of the wire, Si produces a stable effect of deoxidizing weld metal and prevents variation in strength and toughness. Therefore, a preferred total amount of Si in the metal tube and flux should is 0.10-1.40 mass % of the total mass of the wire. Si should be added to flux in the form of alloy such as Fe—Si, Fe—Si—Mn, and Fe—Si—Cr.

Mn: 0.55-1.60 mass %

[0028] Mn deoxidizes weld metal, thereby controlling its strength and toughness, and optimizes the amount of slag-forming agent. In addition, it adjusts the bead shape when added in combination with Si, TiO₂, Mg, and fluorides. Therefore, Mn should be added to either or both of the metal tube and flux. If the total content of Mn in the metal tube and flux is less than 0.55 mass % of the total mass of the wire, Mn does not fully produce its deoxidizing effect but causes weld flaws such as blowhole, resulting in weld metal lacking strength and toughness. If the total content of Mn in the metal tube and flux exceeds 1.60 mass % of the total mass of the wire, Mn makes molten metal to have such an excessive fluidity that the bead shape is too poor to be practical in the case of unfavorable welding position such as vertical and overhead welding. Therefore, the total amount of Mn in the metal tube and flux should be 0.55-1.60 mass % of the total mass of the wire. If the total amount of Mn in the metal tube and flux is less than 1.45 mass % of the total mass of the wire, Mn contributes to a good bead shape. Therefore, a preferred total amount of Mn in the metal tube and flux should be 0.55-1.45 mass % of the total mass of the wire. Mn should be added to flux in the form of simple substance (metallic Mn) or alloy (such as Fe—Mn and Fe—Si—Mn).

Cu: 0.004-0.090 Mass %, Ni: 0.004-0.090 Mass %

[0029] Both Cu and Ni prevent weld metal from fluctuating in strength and toughness. They may be added to either or both of the metal tube and flux. If the total content of Cu or Ni in the metal tube and flux is less than 0.004 mass % of the total mass of the wire, they do not produce the effect of preventing weld metal from fluctuating in strength and toughness. On the other hand, if the total content of Cu or Ni in the metal tube and flux is more than 0.090 mass % of the total mass of the wire, they cause weld metal to decrease in strength. Therefore, the total amount of Cu in the metal tube and flux should be 0.004-0.090 mass % of the total mass of the wire, and the total amount of Ni in the metal tube and flux should be 0.004-0.090 mass % of the total mass of the wire. Cu or Ni should be added to flux in the form of simple substance (Cu metal or Ni metal) or in the form of alloy. In the case of seamless wire, Cu or Ni may be added in the form of plating for surface treatment.

Cr: Less Than 2.60 Mass %, Mo: 0.30-1.20 Mass %

[0030] Both Cr and Mo are added to either or both of the metal tube or flux so that weld metal has controlled strength and toughness. MO increases the resistance to temper softening of weld metal and prevents it from decreasing in strength during PWHT. The amount of these components should be properly adjusted so that the weld metal has the same composition as the base metal to be welded. If the total content of Mo in the metal tube and flux is less than 0.30 mass % of the total mass of the wire, the resulting weld metal does not have sufficient strength and toughness. On the other hand, if the total content of Cr in the metal tube and flux is more than 2.60 mass % of the total mass of the wire, or if the total content of Mo in the metal tube and flux is more than 1.20 mass % of the total mass of the wire, the resulting weld metal has excessively high strength and has extremely poor toughness due to temper embrittlement. Therefore, the total amount of Cr in the metal tube and flux should be less than 2.60 mass % of the total mass of the wire, and the total amount of Mo in the metal tube and flux should be 0.30-1.20 mass % of the total mass of the wire. Cu or Ni should be added to flux in the form of simple substance (Cr metal or Mo metal) or in the form of alloy (such as Fe—Cr and Fe—Mo).

TiO₂: 4.2-8.2 Mass %

[0031] TiO₂ is a major component of the slag-forming agent. It functions also as an arc stabilizer. In addition, it optimizes the amount of the slag-forming agent, and when added in combination with Si, Mn, Mg, and fluorides, it adjusts the bead shape. Therefore, TiO₂ should be added to flux. If the content of TiO₂ is less than 4.2 mass % of the total mass of the wire, the bead shape is poor and the arc stability deteriorates to increase spatter, making welding operation impracticable. On the other hand, if the content of TiO₂ is more than 8.2 mass % of the total mass of the wire, the slag viscosity becomes extremely high, causing weld flaws such as slag inclusion. Therefore, the content of TiO₂ should be 4.2-8.2 mass % of the total mass of the wire.

Metal Fluorides: 0.025-0.25 Mass % in Terms of F

[0032] Metal fluorides function as an arc stabilizer. They also control the viscosity of molten slag and optimizes the amount of slag-forming agent. When added in combination with Si, Mn, TiO₂, and Mg, they adjust the bead shape. Moreover, they dissociate in an arc, thereby liberating fluorine gas which stirs molten metal. The stirring of molten metal promotes floating and separation of slag from molten metal and reduces the amount of oxygen in weld metal. For this reason, they are added to flux. With a content less than 0.025 mass % (in terms of fluorine) of the total mass of the wire, the metal fluorides do not produce the effect of adjusting the bead shape and reducing the amount of oxygen in weld metal. Moreover, metal fluorides in such a content aggravates arc stability, thereby increasing spatter and resulting in a poor bead shape, with the result that the weld metal is poor in toughness. On the other hand, with a content exceeding 0.25 mass % (in terms of fluorine) of the total mass of the wire, the metal fluorides make the slag excessively flowable, adversely affecting the ability of slag to cover molten metal, and aggravate the bead shape. Therefore, the content of metal fluorides should be 0.025-0.25 mass % (in terms of fluorine) of the total mass of the wire. Incidentally, the metal fluorides may be in the form of LiF, NaF, K₂SiF₆, CaF₂, MgF₃, BaF₂, or CeF₃.

Mg: 0.20-1.50 Mass %

[0033] Mg is a strong deoxidizing agent, and it is added to flux to deoxidize weld metal, thereby improving weld metal in toughness. With a content less than 0.20 mass % of the total mass of the wire, Mg does not fully produce its deoxidizing effect but causes blowhole and deteriorates toughness. On the other hand, with a content more than 1.50 mass % of the total mass of the wire, Mg increases the amount of spatter and make molten metal excessively flowable. Mg in such an amount extremely aggravates the bead shape in the case of vertical or overhead welding, even though it optimizes the amount of the slag-forming agent and the respective amounts of Si, Mn, TiO₂, Mg, and fluorides are within the range specified in the present invention. Therefore, the content of Mg should be 0.20-1.50 mass % of the total mass of the wire. Incidentally, Mg may be added in the form of simple substance (such as metallic Mg) or alloys (such as Al—Mg, Si—Mg, and Ni—Mg).

[0034] The foregoing is the fundamental feature of the present invention. Lack of any of such requirements would make it impossible to reconcile good welding maneuverability with weld metal having good mechanical properties such as strength and toughness. Incidentally, the following additional elements increase strength and toughness and ensure stabler strength and toughness.

Nb: 0.005-0.050 Mass %, V: 0.005-0.050 Mass %

[0035] Nb and V are strong carbide-forming elements. When added in an adequate amount, they fix C in the interface between one bead and the other, thereby preventing the coarsening of ferrite grains and the occurrence of ferrite band in weld metal during PWHT, which in turn prevents the deterioration of toughness. Therefore, they may be added to either or both of the metal tube and flux. If its total amount in the metal tube and flux is less than 0.005 mass % of the total mass of the wire, Nb does not produce its effect of preventing toughness from deteriorating. If its total amount in the metal tube and flux is less than 0.005 mass % of the total mass of the wire, V does not produce its effect of preventing toughness from deteriorating. On the other, if its total amount in the metal tube and flux is more than 0.050 mass % of the total mass of the wire, Nb does not produce any additional effect. If its total amount in the metal tube and flux is more than 0.050 mass % of the total mass of the wire, V does not produce any additional effect. Therefore, the total amount of Nb in the metal tube and flux should preferably be 0.005-0.050 mass % of the total mass of the wire. The total amount of V in the metal tube and flux should preferably be 0.005-0.050 mass % of the total mass of the wire. Incidentally, Nb and V may be added to flux in the form of simple substance or alloy (such as Fe—Nb and Fe—V) or as a trace component in a variety of oxides.

B: 0.005-0.020 Mass %

[0036] B improves toughness through refinement of microstructure of weld metal. It may be added to either or both of the metal tube and flux. If its total content in the metal tube and flux is less than 0.005 mass % of the total mass of the wire, B will not fully produce its effect of improving toughness. If its total content in the metal tube and flux is more than 0.020 mass % of the total mass of the wire, B will not produce additional effect. Therefore, the total content of B in the metal tube and flux should be 0.005-0.020 mass % of the total mass of the wire. Incidentally, B may be added in the form of alloy (such as Fe—B and Fe—Si—B) or in the form of oxide (such as B₂O₃).

P: less than 0.015 Mass % (in Total Content) S: less than 0.015 Mass % (in Total Content)

[0037] P and S make weld metal brittle during PWHT. Limiting their amount favorably stabilizes or improves the toughness of weld metal. Therefore, the total amount of P in the metal tube and flux should preferably be less than 0.015 mass % of the total mass of the wire. The total amount of S in the metal tube and flux should preferably be less than 0.015 mass % of the total mass of the wire. Ti: 0.02-0.2 mass % Ti is added to either or both of the metal tube and flux for arc stabilization and deoxidation of weld metal. If its total amount in the metal tube and flux is less than 0.02 mass % of the total mass of the wire, Ti does not fully produce its effect of arc stabilization and deoxidation of weld metal. On the other hand, even though its total amount in the metal tube and flux is more than 0.2 mass % of the total mass of the wire, Ti does not produce additional effect. Therefore, the total amount of Ti in the metal tube and flux should preferably be less than 0.02-0.02 mass % of the total mass of the wire. Incidentally, Ti may be added to flux in the form of simple substance (such as Ti metal) or in the form of alloy (such as Fe—Ti).

[0038] The shielding gas that can be used in the present invention may be any of 100% CO₂, a mixed gas of Ar and C0₂, a mixed gas of Ar and O₂, and a mixed gas of Ar, CO₂, and O₂. The metal tube may be that of mild steel or alloy steel. The cross section of the metal tube is not specifically restricted; it may or may not have a seam. In the case of seamless cross section, the metal wire may be plated with Cu or Ni as one of the elements required in the present invention.

EXAMPLES

[0039] The invention will be described in more detail which reference to the following examples and comparative examples which demonstrate flux-cored wires for gas-shielded arc welding of heat resisting steel.

Example 1

[0040] Metal tubes having the compositions shown in Tables 1 and 2 were prepared. These metal tubes were made into flux-cored wires having the compositions shown in Tables 3 to 18. Incidentally, all the wires have a diameter of 1.2 mm. In Tables 3 to 18, the symbol “<” denotes “less than”, and the amount of metal fluorides is expressed in terms of total F amount. In Tables 9, 10, 13, 14, 17 and 18, the content of TiO₂, Sio₂, Al₂O₃, ZrO₂, and MgO constituting the slag-forming agent, and the content of NaF, K₂SiF₆, CeF₂, and total F are based on the total mass of the wire.

[0041] Test plates were prepared with each sample of the flux-cored wires, and the welding maneuverability of each sample was evaluated. The weld metal of the test plates was examined by radiographic examination and underwent tensile and impact tests after PWHT. FIG. 1 is a sectional view showing the test plate prepared in the example of the present invention.

[0042] The test plate was prepared in the following way. First, a pair of steel plates 1 having a butt groove angle of 45° are arranged. A backing strip 2 is arranged on the back side of the groove. Using one of the flux-cored wires shown in Tables 3 to 18, multi-layer build-up welding (6 layers and 12 passes) is carried out (starting from the front side of the groove) under the conditions shown in Table 19, so that the multi-layer weld metal 3 is formed in the groove. In this way the test plate is prepared. The test plate undergoes PWHT and then tested for the above-mentioned items. Welding of the steel plate 1 was carried out with wires Nos. 1 to 23 and 31 to 55 (conforming to ASTM A387 Gr. 22), wires Nos. 24 to 26 (conforming to ASTM A387, Gr. 11), wires Nos. 27 and 28 (conforming to ASTM A204 Gr. A), and wires Nos. 29 and 30 (conforming to ASTM A387 Gr. 2).

[0043] To evaluate the welding maneuverability, fillet welding (vertical position) was also performed, in addition to flat welding mentioned above, on the steel plate 1 using each sample of the wires shown in Tables 3 to 18 under the conditions shown in Table 20. Arc stability, slag removability, spatter amount, and bead shape were examined.

[0044] The weld metal was rated by radiographic examination as good if the test result meets the requirements of JIS Class 1; otherwise, it was rated as poor. On the other hand, the tensile and impact properties of the weld metal were evaluated according to the requirements specified in AWS A5.29-1998 and JIS Z3318-1991. The acceptable range of tensile and impact properties is shown in Table 21. The acceptable range of tensile and impact properties differ in the case of wires Nos. 1 to 23 and 31 to 55, wires Nos. 24 to 26, wires Nos. 27 and 28, and wires Nos. 29 and 30, because the kind of the steel to be welded is different.

[0045] Impact test was carried out at 0° C. using a specimen having a 2-mm deep V-notch. An average of three measurements was obtained. The chemical composition of weld metal was determined by chemical analysis of a sample taken from the center of weld metal (as shown in FIG. 1) using a drill (10 mm in diameter).

[0046] The results of the tests are shown in Tables 22 to 37. Incidentally, Tables 24, 28, 32, and 36 showing the results of radiant-ray test have the column of remarks in which “HC” denotes the occurrence of hot cracking, “BH” denotes the occurrence of blow hole, and “SI” denotes the occurrence of slag inclusion. In Tables 22, 23, 27, 30, 31, and 35, the symbol “<” means “less than”. TABLE 1 Kind of Steel Composition of steel tube (mass %) steel tube C Si Mn P S Cu Ni Mild A 0.036 0.005 0.200 0.012 0.007 0.013 0.014 steel B 0.020 0.030 0.300 0.010 0.010 0.005 0.005 C 0.010 0.005 0.250 0.004 0.011 0.011 0.012 Cr—Mo D 0.025 0.500 1.140 0.003 0.007 0.012 0.084 steel E 0.031 0.480 1.100 0.007 0.005 0.013 0.031

[0047] TABLE 2 Kind of Steel Composition of steel tube (mass %) steel tube Cr Mo Ti Nb V B N Mild A 0.020 0.005 0.001 0.003 0.001 0.0001 0.0024 steel B 0.013 0.009 0.005 0.004 0.004 0.0001 0.0030 C 0.019 0.002 0.001 0.003 0.001 0.0001 0.0033 Cr—Mo D 1.390 0.480 0.002 0.003 0.003 0.0001 0.0030 steel E 2.440 1.100 0.001 0.003 0.004 0.0001 0.0090

[0048] TABLE 3 Ratio of Slag-forming Wire Steel flux agent Composition of wire (mass %) No. tube (mass %) (mass %) C Si Mn P Example 1 A 13.50 7.02 0.032 0.405 0.851 0.016 No. 2 A 13.50 7.09 0.147 0.172 0.819 0.017 3 A 11.50 6.44 0.068 0.077 0.680 0.014 4 A 15.50 7.86 0.059 1.328 0.635 0.015 5 A 15.50 6.44 0.068 0.165 0.566 0.014 6 A 13.50 7.02 0.055 0.117 1.539 0.014 7 B 11.00 6.61 0.034 0.135 0.653 0.012 8 B 11.50 6.91 0.034 0.140 0.669 0.012 9 A 15.00 7.80 0.074 0.490 0.985 0.014 10  A 15.00 7.61 0.066 1.068 1.069 0.015 11  A 15.00 7.61 0.066 0.551 1.056 0.014 12  A 12.00 6.72 0.061 0.277 0.683 0.014 13  A 16.00 9.50 0.116 0.485 1.058 0.015 14  A 13.00 7.25 0.053 0.273 0.914 0.014 15  A 13.00 7.25 0.063 0.273 0.914 0.014

[0049] TABLE 4 Wire Composition of wire (mass %) No. S Cu Ni Cr Mo Ti Nb V Example 1 0.016 0.011 0.012 2.116 0.925 <0.005 <0.005 <0.005 No. 2 0.016 0.011 0.012 2.277 0.934 <0.005 <0.005 <0.005 3 0.013 0.012 0.012 2.061 0.904 <0.005 0.017 0.020 4 0.013 0.011 0.012 2.185 1.013 <0.005 0.020 0.024 5 0.013 0.012 0.012 2.084 0.904 <0.005 0.017 0.020 6 0.013 0.011 0.012 2.115 0.976 <0.005 0.018 0.021 7 0.015 0.008 0.013 1.977 0.834 <0.005 0.017 0.022 8 0.015 0.014 0.008 2.066 0.872 <0.005 0.018 0.022 9 0.013 0.011 0.012 2.417 0.981 <0.005 0.020 0.024 10  0.013 0.011 0.012 2.201 0.981 <0.005 0.019 0.023 11  0.013 0.011 0.012 2.215 0.981 <0.005 0.019 0.023 12  0.013 0.011 0.012 2.174 0.943 <0.005 0.014 0.016 13  0.013 0.011 0.012 2.091 1.006 <0.005 0.024 0.029 14  0.013 0.011 0.012 2.132 0.997 <0.005 0.019 0.023 15  0.013 0.011 0.012 2.158 0.981 <0.005 0.019 0.022

[0050] TABLE 5 Composition of wire Wire (mass %) Slag-forming gent (mass %) No. B N Mg TiO₂ SiO₂ Al₂O₃ ZrO₂ Example 1 <0.002 0.002 1.079 6.244 0.357 0.042 0.000 No. 2 <0.002 0.002 0.944 6.244 0.388 0.024 0.000 3 0.007 0.002 0.804 5.559 0.335 0.056 0.000 4 0.014 0.002 0.310 6.717 0.442 0.010 0.000 5 0.007 0.002 0.689 5.559 0.335 0.056 0.000 6 0.012 0.002 0.944 6.019 0.387 0.009 0.000 7 0.007 0.003 0.330 5.238 0.439 0.084 0.000 8 0.008 0.003 0.345 5.476 0.459 0.086 0.000 9 0.013 0.002 1.049 6.688 0.430 0.010 0.000 10  0.013 0.002 0.225 6.500 0.428 0.010 0.000 11  0.013 0.002 1.424 6.500 0.428 0.010 0.000 12  0.008 0.002 0.480 4.391 1.503 0.073 0.000 13  0.010 0.002 0.639 8.120 0.389 0.008 0.000 14  0.008 0.002 0.649 6.410 0.380 0.063 0.000 15  0.008 0.002 0.649 6.260 0.176 0.005 0.000

[0051] TABLE 6 Composition of slag-forming agent (mass %) Wire Total No. MgO Others NaF K₂SiF₆ CeF₃ CaF₂ F Example 1 0.000 0.095 0.146 0.094 0.000 0.039 0.134 No. 2 0.066 0.081 0.146 0.134 0.000 0.000 0.135 3 0.000 0.241 0.237 0.000 0.000 0.000 0.107 4 0.000 0.355 0.168 0.154 0.000 0.000 0.156 5 0.000 0.241 0.247 0.000 0.000 0.000 0.107 6 0.000 0.313 0.146 0.134 0.000 0.000 0.135 7 0.000 0.310 0.216 0.038 0.274 0.000 0.194 8 0.000 0.324 0.226 0.034 0.286 0.000 0.203 9 0.000 0.348 0.162 0.149 0.000 0.000 0.151 10  0.000 0.345 0.310 0.000 0.000 0.000 0.140 11  0.000 0.345 0.310 0.000 0.000 0.000 0.140 12  0.000 0.265 0.248 0.000 0.219 0.000 0.175 13  0.000 0.385 0.094 0.000 0491 0.000 0.185 14  0.000 0.291 0.000 0.000 0.097 0.000 0.028 15  0.000 0.253 0.441 0.000 0.108 0.000 0.231

[0052] TABLE 7 Ratio of Slag-forming Wire Steel flux agent Composition of wire (mass %) No. tube (mass %) (mass %) C Si Mn P Example 16 A 15.00 7.61 0.066 1.067 1.069 0.015 No. 17 A 15.50 7.86 0.067 1.103 0.789 0.015 18 A 12.00 6.24 0.061 0.482 0.685 0.014 19 A 16.00 9.74 0.115 0.506 0.974 0.015 20 C 13.50 7.02 0.033 0.346 1.147 0.009 21 C 15.00 7.61 0.044 0.846 0.877 0.009 22 D 13.50 7.02 0.032 0.767 1.096 0.006 23 E 13.50 7.02 0.036 0.741 1.065 0.010 24 A 15.00 7.80 0.057 0.565 1.209 0.014 25 C 15.00 7.80 0.035 0.566 1.251 0.009 26 D 13.50 7.02 0.031 0.758 1.100 0.006 27 A 15.00 7.76 0.046 0.563 1.204 0.014 28 C 15.00 7.76 0.024 0.563 1.247 0.009 29 A 15.00 7.76 0.047 0.576 0.650 0.014 30 C 15.00 7.76 0.024 0.576 0.693 0.009

[0053] TABLE 8 Wire Composition of wire (mass %) No. S Cu Ni Cr Mo Ti Nb V Example 16 0.013 0.011 0.012 2.201 0.943  0.034 0.019 0.022 No. 17 0.013 0.011 0.012 2.212 0.965  0.197 0.019 0.023 18 0.013 0.011 0.012 2.174 0.943 <0.005 0.014 0.016 19 0.013 0.011 0.012 2.091 1.006 <0.005 0.023 0.028 20 0.010 0.010 0.010 2.115 0.881 <0.005 0.018 0.021 21 0.010 0.009 0.010 2.250 1.025 <0.005 0.017 0.020 22 0.012 0.011 0.073 2.042 1.007 <0.005 0.018 0.023 23 0.010 0.012 0.027 2.111 1.036 <0.005 0.018 0.024 24 0.012 0.011 0.012 1.297 0.464 <0.005 0.020 0.024 25 0.010 0.010 0.010 1.296 0.462 <0.005 0.020 0.024 26 0.012 0.011 0.073 1.203 0.500 <0.005 0.018 0.023 27 0.012 0.011 0.012 0.018 0.464 <0.005 0.020 0.024 28 0.010 0.010 0.010 0.017 0.462 <0.005 0.020 0.024 29 0.012 0.011 0.012 0.542 0.464 <0.005 0.020 0.024 30 0.010 0.010 0.010 0.541 0.462 <0.005 0.020 0.024

[0054] TABLE 9 Composition of wire Composition of slag-forming Wire (mass %) agent (mass %) No. B N Mg TiO₂ SiO₂ Al₂O₃ ZrO₂ Example 16 0.013 0.002 0.210 6.211 0.473 0.159 0.101 No. 17 0.014 0.002 0.217 6.418 0.489 0.164 0.104 18 0.008 0.002 0.480 4.391 1.035 0.067 0.000 19 0.010 0.002 0.400 7.935 0.809 0.013 0.000 20 0.012 0.003 0.944 6.019 0.387 0.009 0.000 21 0.013 0.003 0.599 6.743 0.356 0.012 0.000 22 0.012 0.007 0.944 6.019 0.387 0.009 0.000 23 0.012 0.008 0.944 6.019 0.387 0.009 0.000 24 0.013 0.002 1.049 6.688 0.430 0.010 0.000 25 0.013 0.003 1.049 6.688 0.430 0.010 0.000 26 0.012 0.007 0.944 6.019 0.387 0.009 0.000 27 <0.002 0.002 1.049 6.688 0.449 0.004 0.000 28 <0.002 0.003 1.049 6.688 0.449 0.004 0.000 29 <0.002 0.002 1.049 6.688 0.449 0.004 0.000 30 <0.002 0.003 1.049 6.688 0.449 0.004 0.000

[0055] TABLE 10 Composition of slag-forming agent (mass %) Wire Total No. MgO Others NaF K₂SiF₆ CeF₃ CaF₂ F Example 16 0.000 0.339 0.310 0.000 0.000 0.000 0.140 No. 17 0.000 0.351 0.320 0.000 0.000 0.000 0.145 18 0.000 0.264 0.248 0.000 0.219 0.000 0.175 19 0.000 0.381 0.094 0.000 0.491 0.000 0.185 20 0.000 0.313 0.146 0.134 0.000 0.000 0.135 21 0.000 0.183 0.310 0.000 0.000 0.000 0.140 22 0.000 0.313 0.146 0.134 0.000 0.000 0.135 23 0.000 0.313 0.146 0.134 0.000 0.000 0.135 24 0.000 0.348 0.162 0.149 0.000 0.000 0.151 25 0.000 0.348 0.162 0.149 0.000 0.000 0.151 26 0.000 0.313 0.146 0.134 0.000 0.000 0.135 27 0.000 0.289 0.162 0.149 0.000 0.000 0.151 28 0.000 0.289 0.162 0.149 0.000 0.000 0.151 29 0.000 0.289 0.162 0.149 0.000 0.000 0.151 30 0.000 0.289 0.162 0.149 0.000 0.000 0.151

[0056] TABLE 11 Ratio of Slag-forming Wire Steel flux agent Composition of wire (mass %) No. tube (mass %) (mass %) C Si Mn P Comparative 31 A 16.40 8.20 0.211 1.061 1.088 0.017 Example 32 A 11.50 6.33 0.039 0.011 0.935 0.016 No. 33 A 16.00 8.64 0.033 1.468 0.688 0.017 34 A 15.00 8.25 0.053 1.028 0.524 0.017 35 A 15.00 8.25 0.052 0.529 1.673 0.016 36 B 12.00 6.78 0.036 0.256 0.946 0.015 37 A 11.00 6.22 0.048 0.090 0.564 0.016 38 B 11.00 6.61 0.034 0.670 0.670 0.015 39 A 11.00 6.61 0.048 0.117 0.564 0.016 40 A 16.50 8.91 0.076 0.376 1.159 0.017 41 A 15.50 9.15 0.064 0.965 1.262 0.017 42 A 15.50 9.15 0.032 0.534 0.856 0.016 43 A 15.50 9.46 0.032 0.535 0.855 0.017 44 A 15.50 9.61 0.032 0.535 0.855 0.016 45 A 16.00 8.80 0.034 1.010 1.342 0.016

[0057] TABLE 12 Wire Composition of wire (mass %) No. S Cu Ni Cr Mo Ti Nb V Comparative 31 0.016 0.011 0.012 2.045 0.928 <0.005 <0.005 <0.005 Example 32 0.016 0.012 0.012 2.221 0.896 <0.005 <0.005 <0.005 No. 33 0.015 0.011 0.012 1.946 0.906 <0.005 <0.005 <0.005 34 0.015 0.011 0.012 2.017 0.849 <0.005 <0.005 <0.005 35 0.016 0.011 0.012 2.114 0.849 <0.005 <0.005 <0.005 36 0.018 0.004 0.040 2.024 0.947 <0.005 <0.005 <0.005 37 0.016 0.110 0.012 2.115 1.037 <0.005 <0.005 <0.005 38 0.018 0.010 0.004 2.005 0.834 <0.005 <0.005 <0.005 39 0.016 0.012 0.100 1.999 0.831 <0.005 <0.005 <0.005 40 0.015 0.011 0.012 2.655 0.830 <0.005 0.003 0.005 41 0.015 0.011 0.012 2.369 0.276 <0.005 0.003 0.004 42 0.016 0.011 0.012 2.075 1.324 <0.005 0.003 0.004 43 0.016 0.011 0.012 1.998 0.975 <0.005 0.003 0.003 44 0.016 0.011 0.012 1.998 0.975 <0.005 0.003 0.005 45 0.015 0.011 0.012 2.291 0.906 <0.005 0.003 0.005

[0058] TABLE 13 Composition of wire Wire (mass %) Slag-forming agent (mass %) No. B N Mg TiO₂ SiO₂ Al₂O₃ ZrO₂ Compara- 31 <0.002 0.002 1.040 6.914 0.420 0.110 0.000 tive 32 <0.002 0.002 1.034 5.514 0.319 0.074 0.000 Example 33 <0.002 0.002 2.240 7.704 0.289 0.077 0.000 No. 34 <0.002 0.002 0.375 7.233 0.360 0.072 0.000 35 <0.002 0.002 0.524 7.178 0.445 0.075 0.000 36 <0.002 0.003 0.360 5.922 0.292 0.062 0.000 37 <0.002 0.002 0.330 5.429 0.327 0.054 0.000 38 <0.002 0.003 0.330 5.434 0.380 0.086 0.000 39 <0.002 0.002 0.330 5.434 0.440 0.083 0.000 40 <0.000 0.002 1.319 7.615 0.292 0.077 0.000 41 0.000 0.002 0.418 7.154 0.710 0.074 0.520 42 0.000 0.002 0.310 7.154 0.620 0.072 0.520 43 0.000 0.002 0.619 4.119 3.879 0.119 0.822 44 0.000 0.002 0.465 8.315 0.565 0.011 0.302 45 0.000 0.002 0.192 7.385 0.470 0.228 0.000

[0059] TABLE 14 Composition of slag-forming agent (mass %) Wire Total No. MgO Others NaF K₂SiF₆ CeF₃ CaF₂ F Compara- 31 0.321 0.118 0.177 0.000 0.136 0.000 0.120 tive 32 0.067 0.095 0.181 0.000 0.048 0.022 0.107 Example 33 0.172 0.090 0.173 0.000 0.133 0.000 0.117 No. 34 0.000 0.135 0.310 0.149 0.000 0.000 0.217 35 0.000 0.063 0.442 0.045 0.000 0.000 0.223 36 0.000 0.113 0.354 0.036 0.000 0.000 0.179 37 0.000 0.045 0.324 0.033 0.000 0.000 0.164 38 0.000 0.147 0.433 0.033 0.091 0.000 0.239 39 0.000 0.088 0.433 0.033 0.091 0.000 0.239 40 0.186 0.123 0.341 0.000 0.274 0.000 0.233 41 0.000 0.196 0.000 0.231 0.257 0.000 0.194 42 0.000 0.200 0.320 0.000 0.257 0.000 0.219 43 0.000 0.163 0.320 0.000 0.000 0.000 0.145 44 0.000 0.093 0.320 0.000 0.000 0.000 0.145 45 0.000 0.131 0.157 0.080 0.345 0.000 0.212

[0060] TABLE 15 Ratio of Slag-forming Wire Steel flux agent Composition of wire (mass %) No. tube (mass %) (mass %) C Si Mn P Comparative 46 A 15.50 8.37 0.074 0.490 0.830 0.017 Example 47 A 15.50 8.37 0.064 0.709 0.687 0.017 No. 48 A 15.50 8.37 0.064 0.709 0.687 0.017 49 A 11.50 5.96 0.072 0.168 0.609 0.017 50 A 17.00 10.10 0.066 0.631 0.742 0.016 51 A 11.50 6.56 0.039 0.036 0.499 0.016 52 A 15.50 9.75 0.031 0.284 0.854 0.017 53 A 11.50 5.96 0.062 0.042 0.506 0.017 54 A 17.00 10.10 0.066 0.631 0.742 0.017 55 A 16.00 8.64 0.032 1.480 0.438 0.017

[0061] TABLE 16 Wire Composition of wire (mass %) No. S Cu Ni Cr Mo Ti Nb V Comparative 46 0.015 0.011 0.012 2.082 0.877 <0.005 0.003 0.004 Example 47 0.015 0.011 0.012 2.082 0.877 <0.005 0.003 0.004 No. 48 0.015 0.011 0.012 2.082 0.877 <0.005 0.003 0.005 49 0.015 0.012 0.012 2.148 0.919 <0.005 0.003 0.004 50 0.015 0.011 0.012 2.053 0.962 <0.005 0.003 0.005 51 0.015 0.012 0.012 2.221 0.896 <0.005 0.003 0.004 52 0.016 0.011 0.012 1.997 0.975 <0.005 0.003 0.003 53 0.016 0.012 0.012 2.084 0.868 <0.005 0.003 0.004 54 0.015 0.011 0.012 2.053 0.962 <0.005 0.003 0.003 55 0.015 0.011 0.012 2.015 0.906 <0.005 0.003 0.005

[0062] TABLE 17 Composition of wire Wire (mass %) Slag-forming agent (mass %) No. B N Mg TiO₂ SiO₂ Al₂O₃ ZrO₂ Comparative 46 0.000 0.002 1.548 7.154 0.274 0.250 0.000 Example 47 0.000 0.002 1.239 7.154 0.751 0.163 0.000 No. 48 0.000 0.002 1.239 7.316 0.168 0.060 0.000 49 0.000 0.002 0.919 5.285 0.332 0.055 0.000 50 0.000 0.002 1.274 8.152 1.219 0.187 0.000 51 0.000 0.002 1.034 5.514 0.544 0.077 0.000 52 0.000 0.002 0.619 4.119 3.879 0.119 0.882 53 0.000 0.002 0.919 5.285 0.332 0.055 0.000 54 0.000 0.002 1.274 4.076 4.675 0.530 0.000 55 0.000 0.002 0.240 7.704 0.289 0.077 0.000

[0063] TABLE 18 Composition of slag-forming agent (mass %) Wire Total No. MgO Others NaF K₂SiF₆ CeF₃ CaF₂ F Compara- 46 0.000 0.113 0.320 0.000 0.257 0.000 0.219 tive 47 0.000 0.213 0.000 0.000 0.077 0.000 0.022 Example 48 0.000 0.120 0.213 0.092 0.399 0.000 0.260 No. 49 0.000 0.043 0.124 0.114 0.000 0.000 0.115 50 0.000 0.136 0.217 0.169 0.000 0.000 0.186 51 0.067 0.096 0.181 0.000 0.048 0.022 0.107 52 0.000 0.168 0.609 0.000 0.000 0.000 0.276 53 0.000 0.043 0.124 0.114 0.000 0.000 0.115 54 0.000 0.363 0.217 0.169 0.000 0.000 0.186 55 0.172 0.090 0.173 0.000 0.133 0.000 0.177

[0064] TABLE 19 Welding conditions Corresponding wire Welding current 250 A (DCEP) Arc voltage 30-32 V Welding speed 25-30 cm/min Welding position Flat position Shieldubg gas flow 25 liters/min rate Preheating and 176 ± 15° C. Wire Nos. 1 to 26 and 29 to 55 interpass temperature 150 ± 15° C. Wire Nos. 27 and 28

[0065] TABLE 20 Welding conditions Corresponding wire Welding current 250 A (DCEP) Arc voltage 24-26 V Welding speed 20-30 cm/min Welding position Vertical position Shielding gas flow 25 liters/min rate Preheating and 176 ± 15° C. Wire Nos. 1 to 26 and 29 to 55 interpass temperature 150 ± 15° C. Wire Nos. 27 and 28

[0066] TABLE 21 Acceptable tensile properties Acceptable 0.2% proof Tensile impact Wire stress strength Elongation properties Wire Nos. 540 MPa and 630-760 MPa 17% 27 J and above 1 to 23 above and above for average Wire Nos. values and 31 to 55 measured values Wire Nos. 470 MPa and 560-690 MPa 19% 24 to 30 above and above

[0067] TABLE 22 Wire Composition of weld metal (mass %) No. No. Shielding gas C Si Mn P S Cu Example 1 1 100% CO₂ 0.036 0.22 0.48 0.017 0.015 0.019 No. 2 2 100% CO₂ 0.150 0.09 0.46 0.017 0.016 0.019 3 3 80% Ar + 20% CO₂ 0.079 0.23 0.66 0.015 0.013 0.019 4 4 100% CO₂ 0.065 0.71 0.36 0.015 0.013 0.018 5 5 75% Ar + 25% CO₂ 0.079 0.28 0.60 0.015 0.013 0.019 6 6 75% Ar + 25% CO₂ 0.066 0.25 1.14 0.015 0.013 0.019 7 7 80% Ar + 20% CO₂ 0.042 0.26 0.64 0.013 0.015 0.013 8 8 80% Ar + 20% CO₂ 0.043 0.26 0.65 0.013 0.015 0.023 9 9 100% CO₂ 0.083 0.26 0.56 0.015 0.013 0.018 10  10  100% CO₂ 0.073 0.57 0.60 0.015 0.013 0.018 11  11  100% CO₂ 0.079 0.29 0.60 0.015 0.013 0.018 12  12  100% CO₂ 0.067 0.15 0.38 0.015 0.013 0.019 13  13  100% CO₂ 0.130 0.26 0.61 0.015 0.013 0.019 14  14  100% CO₂ 0.070 0.15 0.51 0.015 0.013 0.019 15  15  100% CO₂ 0.070 0.15 0.51 0.015 0.013 0.019

[0068] TABLE 23 Composition of weld metal (mass %) Ni Cr Mo Ti Nb V B N Example 1 0.018 2.15 1.04 0.022 <0.005 <0.005 <0.002 0.004 No. 2 0.018 2.31 1.05 0.022 <0.005 <0.005 <0.002 0.004 3 0.018 2.27 1.01 0.045 0.009 0.022 0.002 0.004 4 0.018 2.24 1.14 0.023 0.007 0.021 0.005 0.004 5 0.018 2.29 1.01 0.043 0.009 0.022 0.002 0.004 6 0.018 2.34 1.09 0.044 0.009 0.024 0.004 0.004 7 0.020 2.19 0.93 0.046 0.009 0.024 0.002 0.005 8 0.012 2.28 0.97 0.047 0.009 0.024 0.003 0.005 9 0.018 2.47 1.11 0.023 0.007 0.021 0.004 0.004 10  0.018 2.25 1.10 0.023 0.007 0.020 0.004 0.004 11  0.018 2.26 1.10 0.022 0.007 0.020 0.004 0.004 12  0.018 2.20 1.05 0.018 0.005 0.014 0.003 0.004 13  0.018 2.18 1.16 0.025 0.008 0.026 0.003 0.004 14  0.018 2.17 1.12 0.022 0.007 0.020 0.003 0.004 15  0.018 2.19 1.10 0.022 0.006 0.020 0.003 0.004

[0069] TABLE 24 Rating of Conditions of welding PWHT (° C. × maneuver- Radiographic examination hr, cooled No. ability Rating Note in furnace) Exam- 1 Good Good JIS Class 1 — 690 × 1 ple 2 Good Good JIS Class 1 — 690 × 1 No. 3 Good Good JIS Class 1 — 690 × 1 4 Good Good JIS Class 1 — 690 × 1 5 Good Good JIS Class 1 — 690 × 1 6 Good Good JIS Class 1 — 690 × 1 7 Good Good JIS Class 1 — 690 × 1 8 Good Good JIS Class 1 — 690 × 1 9 Good Good JIS Class 1 — 690 × 1 10  Good Good JIS Class 1 — 690 × 1 11  Good Good JIS Class 1 — 690 × 1 12  Good Good JIS Class 1 — 690 × 1 13  Good Good JIS Class 1 — 690 × 1 14  Good Good JIS Class 1 — 690 × 1 15  Good Good JIS Class 1 — 690 × 1

[0070] TABLE 25 Results of tensile test Tensile 0.2% proof strength Elonga- Results of impact test No. stress (MPa) (MPa) tion (%) Measured values (J) Average (J) Example 1 553 638 23 35 59 64 53 No. 2 599 669 22 32 47 73 51 3 663 738 24 52 72 98 74 4 635 694 19 53 64 85 67 5 680 748 24 52 59 89 67 6 627 710 22 52 61 99 71 7 639 709 25 54 64 81 66 8 683 747 25 51 55 96 67 9 634 696 19 50 64 97 70 10  632 691 19 55 74 89 73 11  580 655 20 50 61 93 68 12  567 622 22 55 69 82 69 13  594 703 23 51 73 90 71 14  541 632 25 59 66 72 66 15  559 640 24 56 69 82 69

[0071] TABLE 26 Wire Composition of weld metal (mass %) No. No. Shielding gas C Si Mn P S Cu Example 16 16 100% CO₂ 0.073 0.57 0.60 0.015 0.013 0.018 No. 17 17 100% CO₂ 0.075 0.59 0.45 0.015 0.013 0.018 18 18 100% CO₂ 0.067 0.25 0.38 0.015 0.013 0.019 19 19 100% CO₂ 0.131 0.28 0.56 0.016 0.013 0.019 20 20 80% Ar + 20% CO₂ 0.041 0.37 0.92 0.009 0.010 0.016 21 21 100% CO₂ 0.049 0.45 0.49 0.009 0.010 0.016 22 22 100% CO₂ 0.035 0.41 0.61 0.007 0.012 0.018 23 23 100% CO₂ 0.040 0.39 0.60 0.010 0.010 0.019 24 24 100% CO₂ 0.063 0.30 0.68 0.015 0.013 0.019 25 25 100% CO₂ 0.039 0.30 0.71 0.010 0.010 0.016 26 26 100% CO₂ 0.034 0.40 0.62 0.006 0.012 0.018 27 27 100% CO₂ 0.051 0.30 0.68 0.015 0.013 0.019 28 28 100% CO₂ 0.027 0.30 0.70 0.009 0.010 0.016 29 29 100% CO₂ 0.052 0.31 0.37 0.015 0.013 0.019 30 30 100% CO₂ 0.027 0.31 0.39 0.010 0.010 0.016

[0072] TABLE 27 Composition of weld metal (mass %) No. Ni Cr Mo Ti Nb V B N Ex- 16 0.018 2.25 1.08 0.041 0.006 0.019 0.004 0.004 am- 17 0.018 2.26 1.09 0.096 0.007 0.020 0.005 0.004 ple 18 0.018 2.19 1.05 0.018 0.005 0.014 0.003 0.004 No. 19 0.018 2.18 1.16 0.025 0.008 0.025 0.003 0.004 20 0.016 2.33 0.99 0.045 0.009 0.024 0.004 0.006 21 0.015 2.30 1.15 0.023 0.006 0.011 0.004 0.006 22 0.109 2.07 1.13 0.022 0.006 0.020 0.004 0.014 23 0.040 2.14 1.16 0.022 0.006 0.021 0.004 0.016 24 0.018 1.33 0.52 0.023 0.007 0.021 0.004 0.004 25 0.015 1.33 0.52 0.023 0.007 0.021 0.004 0.006 26 0.109 1.22 0.56 0.022 0.006 0.020 0.004 0.015 27 0.018 0.02 0.52 0.023 0.007 0.021 <0.002  0.004 28 0.015 0.02 0.52 0.023 0.007 0.021 <0.002  0.006 29 0.018 0.55 0.52 0.023 0.007 0.021 <0.002  0.004 30 0.015 0.55 0.52 0.023 0.007 0.021 <0.002  0.006

[0073] TABLE 28 Rating of Conditions of welding PWHT (° C. × maneuver- Radiographic examination hr, cooled No. ability Rating Note in furnace) Exam- 16 Good Good JIS Class 1 — 690 × 1 ple 17 Good Good JIS Class 1 — 690 × 1 No. 18 Good Good JIS Class 1 — 690 × 1 19 Good Good JIS Class 1 — 690 × 1 20 Good Good JIS Class 1 — 690 × 1 21 Good Good JIS Class 1 — 690 × 1 22 Good Good JIS Class 1 — 690 × 1 23 Good Good JIS Class 1 — 690 × 1 24 Good Good JIS Class 1 — 690 × 1 25 Good Good JIS Class 1 — 690 × 1 26 Good Good JIS Class 1 — 690 × 1 27 Good Good JIS Class 1 — 690 × 1 28 Good Good JIS Class 1 — 690 × 1 29 Good Good JIS Class 1 — 690 × 1 30 Good Good JIS Class 1 — 690 × 1

[0074] TABLE 29 Results of tensile test Tensile 0.2% proof strength Elonga- Results of impact test No. stress (MPa) (MPa) tion (%) Measured values (J) Average (J) Example 16 655 705 19 78 96 139 104  No. 17 674 724 19 79 96 131 102  18 568 636 22 56 54 97 69 19 595 702 24 52 75 87 71 20 681 735 23 52 76 91 73 21 551 639 19 53 81 89 74 22 573 651 26 54 75 98 76 23 554 625 24 55 71 92 73 24 563 634 23 60 69 77 69 25 593 653 24 52 67 87 69 26 596 656 23 65 77 98 80 27 551 638 28 43 68 72 61 28 535 623 27 45 65 75 62 29 575 645 24 45 68 76 63 30 548 639 24 47 69 71 62

[0075] TABLE 30 Wire Composition of weld metal (mass %) No. No. Shielding gas C Si Mn P S Cu Compara- 54 31 100% CO₂ 0.235 0.57 0.61 0.017 0.016 0.018 tive 55 32 80% Ar + 20% CO₂ 0.047 0.09 0.31 0.016 0.016 0.019 Example 56 33 100% CO₂ 0.037 0.79 0.39 0.017 0.016 0.018 No. 57 34 80% Ar + 20% CO₂ 0.064 0.71 0.29 0.017 0.015 0.019 58 35 100% CO₂ 0.058 0.29 0.94 0.017 0.016 0.019 59 36 75% Ar + 25% CO₂ 0.044 0.29 0.53 0.015 0.018 0.007 60 37 80% Ar + 20% CO₂ 0.057 0.20 0.42 0.017 0.016 0.182 61 38 80% Ar + 20% CO₂ 0.037 0.23 0.48 0.015 0.018 0.016 62 39 80% Ar + 20% CO₂ 0.057 0.21 0.42 0.017 0.016 0.019 63 40 100% CO₂ 0.085 0.20 0.65 0.017 0.015 0.018 64 41 100% CO₂ 0.072 0.52 0.71 0.017 0.015 0.019 65 42 100% CO₂ 0.036 0.29 0.48 0.017 0.016 0.019 66 43 100% CO₂ 0.036 0.29 0.48 0.017 0.016 0.019 67 44 100% CO₂ 0.036 0.29 0.48 0.017 0.016 0.019 68 45 100% CO₂ 0.038 0.55 0.75 0.017 0.015 0.018

[0076] TABLE 31 Composition of weld metal (mass %) No. Ni Cr Mo Ti Nb V B N Comparative 54 0.018 2.10 1.05 0.023 <0.005 <0.005 <0.002 0.004 Example 55 0.018 2.43 0.99 0.042 <0.005 <0.005 <0.002 0.004 No. 56 0.018 2.01 1.03 0.025 <0.005 <0.005 <0.002 0.004 57 0.018 2.26 0.96 0.046 <0.005 <0.005 <0.002 0.004 58 0.018 2.17 0.96 0.024 <0.005 <0.005 <0.002 0.004 59 0.060 2.24 1.06 0.025 <0.005 <0.005 <0.002 0.005 60 0.019 2.32 1.15 0.041 <0.005 <0.005 <0.002 0.004 61 0.007 2.02 0.93 0.025 <0.005 <0.005 <0.002 0.005 62 0.150 2.21 0.93 0.043 <0.005 <0.005 <0.002 0.004 63 0.018 2.75 0.95 0.024 <0.005 <0.005 <0.002 0.004 64 0.018 2.46 0.32 0.024 <0.005 <0.005 <0.002 0.004 65 0.018 2.15 1.52 0.023 <0.005 <0.005 <0.002 0.004 66 0.018 2.08 1.12 0.018 <0.005 <0.005 <0.002 0.004 67 0.018 2.08 1.12 0.026 <0.005 <0.005 <0.002 0.004 68 0.018 2.37 1.03 0.024 <0.005 <0.005 <0.002 0.004

[0077] TABLE 32 Rating of welding Conditions of maneuver- Radiographic examination PWHT (° C. × hr, No. ability Rating Note cooled in furnace) Comparative 54 Good Poor JIS Class 1 under HG, BH 690 × 1 Example 55 Poor Poor JIS Class 1 under BH 690 × 1 No. 56 Poor Good JIS Class 1 — 690 × 1 57 Poor Poor JIS Class 1 under BH 690 × 1 58 Poor Good JIS Class 1 — 690 × 1 59 Good Good JIS Class 1 — 690 × 1 60 Good Good JIS Class 1 — 690 × 1 61 Good Good JIS Class 1 — 690 × 1 62 Good Good JIS Class 1 — 690 × 1 63 Good Good JIS Class 1 — 690 × 1 64 Good Good JIS Class 1 — 690 × 1 65 Good Good JIS Class 1 — 690 × 1 66 Poor Good JIS Class 1 — 690 × 1 67 Poor Poor JIS Class 1 under Si 690 × 1 68 Poor Poor JIS Class 1 under BH 690 × 1

[0078] TABLE 33 Results of tensile test Tensile 0.2% proof strength Elonga- Results of impact test No. stress (MPa) (MPa) tion (%) Measured values (J) Average (J) Comparative 54 Not evaluated Not evaluated — Example 55 530 603 21  7 18 25 17 No. 56 586 645 17  7  8  8  8 57 535 629 20 12 18 21 17 58 558 662 23 32 42 63 46 59 585 672 21  7 15 68 30 60 534 615 19 28 34 69 44 61 552 642 23  6 12 60 26 62 535 618 24 28 28 61 39 63 699 773 16  9 10 13 11 64 533 615 16 16 19 23 19 65 683 765 15 17 21 23 20 66 598 663 22 47 59 76 61 67 560 639 17 32 61 75 56 68 578 643 22  7  7 11  8

[0079] TABLE 34 Wire Composition of weld metal (mass %) No. No. Shielding gas C Si Mn P S Cu Comparative 69 46 100% CO₂ 0.083 0.26 0.49 0.017 0.015 0.018 Example 70 47 100% CO₂ 0.071 0.38 0.38 0.017 0.015 0.018 No. 71 48 100% CO₂ 0.071 0.38 0.38 0.017 0.015 0.018 72 49 75% Ar + 25% CO₂ 0.083 0.24 0.44 0.017 0.016 0.019 73 50 100% CO₂ 0.075 0.34 0.42 0.017 0.016 0.018 74 51 100% CO₂ 0.042 0.02 0.28 0.016 0.016 0.019 75 52 100% CO₂ 0.036 0.15 0.48 0.017 0.016 0.019 76 53 100% CO₂ 0.068 0.02 0.28 0.017 0.016 0.019 77 54 100% CO₂ 0.075 0.34 0.42 0.017 0.016 0.018 78 55 100% CO₂ 0.036 0.80 0.25 0.017 0.016 0.018

[0080] TABLE 35 Composition of weld metal (mass %) No. Ni Cr Mo Ti Nb V B N Comparative 69 0.018 2.14 1.00 0.023 <0.005 <0.005 <0.002 0.004 Example 70 0.018 2.14 1.00 0.024 <0.005 <0.005 <0.002 0.004 No. 71 0.018 2.14 1.00 0.024 <0.005 <0.005 <0.002 0.004 72 0.018 2.34 1.02 0.043 <0.005 <0.005 <0.002 0.004 73 0.018 2.15 1.11 0.025 <0.005 <0.005 <0.002 0.004 74 0.018 2.24 1.00 0.020 <0.005 <0.005 <0.002 0.004 75 0.018 2.09 1.12 0.018 <0.005 <0.005 <0.002 0.004 76 0.018 2.09 0.96 0.020 <0.005 <0.005 <0.002 0.004 77 0.018 2.15 1.11 0.018 <0.005 <0.005 <0.002 0.004 78 0.018 2.08 1.03 0.025 <0.005 <0.005 <0.002 0.004

[0081] TABLE 36 Rating of welding Conditions of maneuver- Radiographic examination PWHT (° C. × hr, No. ability Rating Note cooled in furnace) Comparative Example No. 69 Poor Good JIS Class 1 — 690 × 1 70 Poor Poor JIS Class 1 under BH, SI 690 × 1 71 Poor Good JIS Class 1 — 690 × 1 72 Poor Poor JIS Class 1 under SI 690 × 1 73 Poor Poor JIS Class 1 under SI 690 × 1 74 Poor Poor JIS Class 1 under BH 690 × 1 75 Poor Good JIS Class 1 — 690 × 1 76 Poor Poor JIS Class 1 under BH, SI 690 × 1 77 Poor Poor JIS Class 1 under SI 690 × 1 78 Poor Poor JIS Class 1 under BH 690 × 1

[0082] TABLE 37 Results of tensile test Tensile Results 0.2% proof strength Elonga- of impact test No. stress (MPa) (MPa) tion (%) Measured values (J) Average (J) Comparative 69 564 638 23 45 59 93 66 Example 70 572 648 23 11 18 18 16 No. 71 578 645 23 82 49 65 49 72 635 715 22 29 42 61 44 73 625 703 21 32 49 58 46 74 528 625 13  7  7  9  8 75 560 652 22 29 31 45 35 76 531 607 11 10 12 15 12 77 608 675 22 34 42 42 39 78 521 608 11  6  7  9  7

[0083] As Tables 22 to 37 above show, those samples in Examples 1 to 30 contain as much slag-forming agent, C, Si, Mn, Cu, Ni, Cr, Mo, TiO₂, metal fluoride, and Mg as specified in Claim 1. Therefore, they are superior in welding maneuverability, radiographic examination, tensile properties, and impact properties, and they meet the requirements for acceptance as shown in Table 21. Samples in Examples 3 to 30 contain P and S in an amount less than 0.015 mass %; samples in n Examples 3 to 26 contain B in an amount of 0.05-0.020 mass %; and samples in Examples 16 and 17 contain Ti in an amount of 0.0002-0.3 mass %. These samples are classified into the following four groups.

[0084] Group (1): Wire Nos. 1 and 2 (P and S not limited, B not added Ti not added)

[0085] Group (2): Wire Nos. 27 to 30 (P and S limited, B not added, Ti not added)

[0086] Group (3): Wire Nos. 3 to 26 (P and S limited, B added, Ti not added)

[0087] Group (4): Wire Nos. 16 and 17 (P and S limited, B added, Ti added)

[0088] It was found that samples vary in impact values from one group to another in the order of (1)<(2)<(3)<(4). In other words, samples in Examples 16 and 17 (as specified in claim 3) have the best characteristics.

[0089] On the other hand, sample No. 54 (in Comparative Example) contains C in an amount exceeding the upper limit specified in the present invention and hence it suffered hot cracking and blowhole in weld metal on account of C although it keeps its welding maneuverability unaffected. This hot cracking was long enough to be visible immediately after welding. Therefore, this sample underwent only radiographic examination but did not undergo tensile test and impact test.

[0090] Sample No. 55 (in Comparative Example) contains Si in an amount less than the lower limit specified in the invention; therefore, it gave weld metal having insufficient viscosity which in turn resulted in a bead with an undesirable convex shape in the case of vertical welding. Moreover, it suffered blowhole due to insufficient deoxidation and hence the resulting weld metal had tensile properties and impact properties which do not meet the requirements for acceptance (as shown Table 21). Sample No. 56 (in Comparative Example) contains Si in an amount more than the upper limit specified in the invention; therefore, it gave weld metal having insufficient viscosity which in turn resulted in a bead with an undesirable convex shape in the case of vertical welding. On the other hand, weld metal had no weld defects detectable by radiographic examination but it became brittle after PWHT and hence it gave impact properties which do not meet the requirements for acceptance (as shown Table 21).

[0091] Sample No. 57 (in Comparative Example) contains Mn in an amount less than the lower limit specified in the invention; therefore, it gave weld metal which resulted in a bead with an undesirable convex shape in the case of vertical welding. In addition, it gave weld metal which suffered blowhole and failed radiographic examination due to insufficient deoxidation and which had tensile properties and impact properties which do not meet the requirements for acceptance (as shown Table 21). Sample No. 58 (in Comparative Example) contains Mn in an amount more than the upper limit specified in the invention; therefore, it gave weld metal having excessive fluidity which resulted in a bead with an undesirable convex shape in the case of vertical welding. In addition, it gave weld metal which passed radiographic examination and had tensile properties and impact properties which meet the requirements for acceptance (as shown Table 21).

[0092] Sample No. 59 (in Comparative Example) contains Cu in an amount less than the lower limit specified in the invention. Therefore, it gave weld metal which greatly varied in impact properties and lacked stability in impact properties, although it posed no problems with welding maneuverability and radiographic examination. Sample No. 60 (in Comparative Example) contains Cu in an amount more than the upper limit specified in the invention. Therefore, it gave weld metal which had tensile properties not meeting the requirements for acceptance (as shown Table 21), although it gave weld metal having no problems with impact properties.

[0093] Sample No. 61 (in Comparative Example) contains Ni in an amount less than the lower limit specified in the invention. Therefore, it gave weld metal which greatly varied in impact properties and lacked stability in impact properties. Sample No. 62 (in Comparative Example) contains Ni in an amount more than the upper limit specified in the invention. Therefore, it gave weld metal which had tensile properties not meeting the requirements for acceptance (as shown Table 21), although it posed no problems with welding maneuverability, impact properties, and radiographic examination.

[0094] Sample No. 63 (in Comparative Example) contains Cr in an amount more than the upper limit specified in the invention. Therefore, it gave weld metal which had excessively high strength not meeting the requirements for acceptance (as shown Table 21). In addition, it gave weld metal which had poor toughness due to embrittlement, although it posed no problems with welding maneuverability and radiographic examination.

[0095] Sample No. 64 (in Comparative Example) contains Mo in an amount less than the lower limit specified in the invention. Therefore, it gave weld metal which had tensile properties and impact properties not meeting the requirements for acceptance (as shown Table 21), although it posed no problems with welding maneuverability and radiographic examination. Sample No. 65 (in Comparative Example) contains Mo in an amount more than the upper limit specified in the invention. Therefore, it gave weld metal which had excessively high strength not meeting the requirements for acceptance (as shown Table 21). In addition, it gave weld metal which had poor toughness due to embrittlement, although it posed no problems with welding maneuverability and radiographic examination.

[0096] Sample No. 66 (in Comparative Example) contains TiO₂ in an amount less than the lower limit specified in the invention. Therefore, it caused poor arc stability and much spatter, giving a convex bead in the case of vertical welding, although it posed no problems with welding maneuverability and radiographic examination. Sample No. 67 (in Comparative Example) contains TiO₂ in an amount more than the upper limit specified in the invention. Therefore, it gave a convex bead in the case of vertical welding and caused slag inclusion and the result of radiographic examination was poor.

[0097] Sample No. 68 (in Comparative Example) contains Mg in an amount less than the lower limit specified in the invention. Therefore, it gave a convex bead in the case of vertical welding. In addition, it gave weld metal which suffered blowhole on account of insufficient deoxidation and hence was poor in the results of radiographic examination. It gave weld metal which had impact properties not meeting the requirements for acceptance as shown in Table 21.

[0098] Sample No. 69 (in Comparative Example) contains Mg in an amount more than the upper limit specified in the invention. Therefore, it caused a large amount of spatter and gave a convex bead in the case of vertical welding because of weld metal having excessive fluidity, although it posed no problems with tensile properties and impact properties.

[0099] Sample No. 70 (in Comparative Example) contains fluorides (in terms of F) in an amount less than the lower limit specified in the invention. Therefore, it was poor in arc stability and caused a convex bead in the case of vertical welding because it was unable to control the viscosity of molten slag. It did not permit slag to float on and separate from molten metal and hence it gave weld metal which had impact properties not meeting the requirements for acceptance as shown in Table 21 on account of insufficient deoxidation. In addition, it caused blowhole and slag inclusion and poor results in radiographic examination, although it posed no problems with tensile properties.

[0100] Sample No. 71 (in Comparative Example) contains fluorides (in terms of F) in an amount more than the upper limit specified in the invention. Therefore, it gave excessive fluid flag which in turn causes a convex bead in the case of vertical welding, although it caused no problems with tensile properties and impact properties and the results of radiographic examination.

[0101] Sample No. 72 (in Comparative Example) contains the slag-forming agent in an amount less than the lower limit specified in the invention. Therefore, it gave a poor-looking bead and a convex bead on account of insufficient slag in the case of vertical welding. It also gave weld metal which was poor in slag removability and suffered partial burn-on. Thus, the resulting weld metal caused slag inclusion and gave poor results in radiographic examination, although it posed no problems with tensile properties and impact properties.

[0102] Sample No. 73 (in Comparative Example) contains the slag-forming agent in an amount more than the lower limit specified in the invention. Therefore, it gave a convex bead in the case of vertical welding. It also gave weld metal which was invariably covered by slag during welding and hence caused slag inclusion and gave poor results in radiographic examination, although it posed no problems with tensile properties and impact properties.

[0103] Sample No. 74 (in Comparative Example) contains both Si and Mn in an amount less than the lower limit specified in the invention. Therefore, it gave a convex bead. In addition, it gave weld metal which suffered blowhole on account of insufficient deoxidation and hence was poor in the results of radiographic examination. It gave weld metal which had tensile and impact properties not meeting the requirements for acceptance as shown in Table 21.

[0104] Sample No. 75 (in comparative Example) contains TiO₂ in an amount less than the upper limit specified in the invention and also contains fluorides in an amount more than the upper limit specified in the invention. Therefore, it caused a large amount of spatter and gave a convex bead in the case of vertical welding, although it posed no problems with tensile properties and impact properties and the results of radiographic examination.

[0105] Sample No. 76 (in Comparative Example) contains both Si and Mn in an amount less than the lower limit specified in the invention. Therefore, it gave a convex bead in the case of vertical welding on account of molten metal having insufficient viscosity. It also gave weld metal which was poor in slag removability and suffered partial burn-on and slag inclusion. Thus, the resulting weld metal was poor in the results of radiographic examination and suffered blow-hole due to insufficient deoxidation. The resulting weld metal did not meet the requirements for tensile properties and impact properties required for acceptance specified in Table 21.

[0106] Sample No. 77 (in Comparative Example) contains the slag-forming agent in an amount more than the upper limit specified in the invention and also contains TiO₂ in an amount less than the lower limit specified in the invention. Therefore, it caused poor arc stability and much spatter, giving a convex bead in the case of vertical welding, and gave rise to an excessive amount of slag which preceded weld metal during welding, causing slag inclusion. The results of radiographic examination was poor.

[0107] Sample No. 78 (in Comparative Example) contains Si in an amount more than the upper limit specified in the invention and also contains Mn in an amount less than the lower limit specified in the invention. Therefore, it gave a convex bead in the case of vertical welding. The resulting weld metal suffered remarkable embrittlement and hence was poor in impact properties on account of excessive Si. Moreover, the resulting weld metal suffered blowhole due to insufficient Mn content and hence insufficient deoxidation. The weld metal was poor in the results of radiographic examination and has tensile properties and impact properties not meeting the requirements for acceptance specified in Table 21.

EXAMPLE 2

[0108] Test plates as shown in FIG. 1 were prepared in the same way as in Example 1 by using the flux-cored wires numbered 1 to 23 in Example 1. They underwent PWHT and then examination for ferrite band and tensile and impact tests. PWHT consisted of keeping the sample at 690° C. for 19 hours and cooling it in a furnace. Tensile and impact tests were the same as those in Example 1.

[0109] Examination for ferrite band was carried out in the following way. After PWHT, the weld metal of the sample was cut at equal intervals perpendicular to the weld line so as to take six specimens for observation of sectional microstructure. The specimen, with its surface polished and etched, was inspected under an optical microscope for ferrite band. The sample was rated according to the ratio of occurrence of ferrite band (segregation) calculated by the following formula.

Ratio of occurrence=(Number of cross sections in which ferrite band occurred)/6

[0110] The sample was regarded as acceptable if the ratio of occurrence is less than 33%; otherwise, it was regarded as rejectable. The results are shown in Tables 38 to 41. TABLE 38 PWHT (° C. × Ferrite band Wire hours, cooled in Ratio of No. No. Shielding gas furnace) Rating occurrence Example 31 1 100% CO₂ 690 × 19 Acceptable 16% No. 32 2 100% CO₂ 690 × 19 Acceptable 16% 33 3 80% Ar + 20% CO₂ 690 × 19 Acceptable Not occurred 34 4 100% CO₂ 690 × 19 Acceptable Not occurred 35 5 75% Ar + 25% CO₂ 690 × 19 Acceptable Not occurred 36 6 75% Ar + 25% CO₂ 690 × 19 Acceptable Not occurred 37 7 80% Ar + 20% CO₂ 690 × 19 Acceptable Not occurred 38 8 80% Ar + 20% CO₂ 690 × 19 Acceptable Not occurred 39 9 100% CO₂ 690 × 19 Acceptable Not occurred 40 10  100% CO₂ 690 × 19 Acceptable Not occurred 41 11  100% CO₂ 690 × 19 Acceptable Not occurred 42 12  100% CO₂ 690 × 19 Acceptable Not occurred 43 13  100% CO₂ 690 × 19 Acceptable Not occurred 44 14  100% CO₂ 690 × 19 Acceptable Not occurred 45 15  100% CO₂ 690 × 19 Acceptable Not occurred

[0111] TABLE 39 Results of tensile test Tensile Results 0.2% proof strength Elonga- of impact test No. stress (MPa) (MPa) tion (%) Measured values (J) Average (J) Example 31 463 545 20 65 75  79 73 No. 32 502 571 19 65 78  92 78 33 555 630 21 69 98 102 90 34 532 593 17 74 76 105 85 35 570 639 21 60 75 110 82 36 525 606 19 65 97 107 90 37 535 606 22 65 78 102 82 38 572 638 22 68 88 110 89 39 531 594 17 61 97 115 91 40 529 591 17 63 98 114 92 41 486 559 18 67 82  93 81 42 475 531 19 67 87 105 86 43 498 600 20 72 82 118 91 44 453 540 22 73 89 109 90 45 468 547 21 65 81 105 84

[0112] TABLE 40 PWHT (° C. × Ferrite band Wire Shielding hours, cooled in Ratio of No. No. gas furnace) Rating occurrence Example No. 46 16 100% CO₂ 690 × 19 Acceptable Not occurred 47 17 100% CO₂ 690 × 19 Acceptable Not occurred 48 18 100% CO₂ 690 × 19 Acceptable Not occurred 49 19 100% CO₂ 690 × 19 Acceptable Not occurred 50 20 80% Ar + 690 × 19 Acceptable Not occurred 20% CO₂ 51 21 100% CO₂ 690 × 19 Acceptable Not occurred 52 22 100% CO₂ 690 × 19 Acceptable Not occurred 53 23 100% CO₂ 690 × 19 Acceptable Not occurred

[0113] TABLE 41 Results of tensile test Tensile Results 0.2% proof strength Elonga- of impact test stress (MPa) (MPa) tion (%) Measured values (J) Average (J) Example 46 549 602 17 105  129 145 126  No. 47 565 618 17 100  125 138 121  48 476 543 19 91  99 105 98 49 498 600 21 78  89 110 92 50 570 628 20 67 102 113 74 51 462 546 17 67 112 112 97 52 480 556 23 62  85 112 86 53 464 534 21 65 105 110 93

[0114] As Tables 38 to 41 above show, samples Nos. 31 to 53 in Example had the occurrence of ferrite bands within acceptable limits. Particularly, samples Nos. 33 to 53 incorporated with Nb and V had no occurrence of ferrite bands, indicating their stable microstructure as well as stable tensile and impact properties.

Effect of the Invention

[0115] As mentioned above, the flux-cored wire of the present invention has both good welding maneuverability and ability to give weld metal with good mechanical properties (such as strength and toughness) because of the specific composition and content of the additives added to either or both of its metal tube and flux. 

What is claimed is:
 1. A flux-cored wire for gas-shielded arc welding of heat-resisting steel in the form of a steel tube filled with a flux, which is characterized in that the content of slag-forming agent is 6.10-9.90 mass % (based on the total mass of the wire), said steel tube and said flux all together contain less than 0.20 mass % C, 0.06-1.40 mass % Si, 0.55-1.60 mass % Mn, 0.004-0.090 mass % Cu, 0.004-0.090 mass % Ni, less than 2.60 mass % Cr, and 0.3-1.20 mass % Mo (based on the total mass of the wire), and said flux contains 4.2-8.2 mass % TiO₂, 0.025-0.25 mass % of metal fluoride (in terms of fluorine), and 0.20-1.50 mass % Mg (in terms of fluorine).
 2. A flux-cored wire for gas-shielded arc welding of heat-resisting steel as defined in claim 1, wherein said steel tube and said flux all together further contain 0.005-0.050 mass % Nb, 0.005-0.050 mass % V, and 0.005-0.020 mass % B (based on the total mass of the wire), and said steel tube and said flux contain less than 0.015 mass % P and less than 0.015 mass % S.
 3. A flux-cored wire for gas-shielded arc welding of heat-resisting steel as defined in claim 1, wherein said steel tube and said flux all together further contain 0.02-0.2 mass % Ti (based on the total mass of the wire). 